Water treatment systems such as water softeners and deionizers are used in both industrial and residential applications. In the case of residential applications, water treatment units are typically small in size and include one or two resin tanks which treat the incoming water. When a single resin tank is used, water service to the household is usually interrupted when regeneration is needed. In a two tank treatment system continuous water service is provided by having one tank on-line at all times. The other tank is regenerated and maintained off-line until the first tank is exhausted and requires regeneration.
Water treatment units used in industrial applications usually operate on the same principal as residential treatment units. However, for at least some industrial applications, the quantity of treated water needed mandates either a much larger resin tank or a multiplicity of tanks in order to support the required flow rate.
When a single resin tank is used in an industrial application, it normally must be very large in order to provide sufficient service time before requiring regeneration. When regeneration is required, the flow of regeneration chemistry and rinse water out of the tank can be substantial and may require the need for even larger equipment to treat the regeneration influent. In a single vessel system, the required treatment equipment for the regeneration influent can be very costly and remains idle most of the time.
U.S. Pat. No. 5,069,779, which is hereby incorporated by reference, is owned by the assignee of the subject application and discloses a large scale water treatment system. The system disclosed in this patent includes multiple cation and anion tanks which are regenerated using fresh regeneration solutions periodically. The cation resins are typically generated using an acid based solution. The anion resins are typically regenerated with a caustic solution, such as a sodium hydroxide solution. In order to produce highly purified deionized water from cation and anion exchange units, it is important that the lower portions of the resin beds not become contaminated with unwanted ions, such as metal ions. For this reason, virgin acid and caustic solutions are used, in a typical application, to regenerate exhausted cation and anion resins.
Many industrial processes produce waste acid or caustic solutions. As an example, in the manufacture printed circuit boards, various acid solutions are used which, during processing, become contaminated with copper, sodium and other ions. Relatively inexpensive recovery methods can be used to, at least partially remove these contaminating ions. However, it is usually not cost effective to remove all contaminating ions to produce a virgin acid solution suitable for regeneration in systems where high quality deionized water is needed.
It is desirable to be able to use partially purified acid or caustic solutions in the regeneration of a cation and anion resin used in industrial applications in order to reduce chemical costs by reducing the need for fresh chemicals and to reduce the discharge of salts to the environment. It is also considered desirable to reuse regeneration solutions at least for part of a regeneration cycle.